This invention relates to a lighting panel for use in a lighting fixture having linear light sources. In particular, it relates to an improved lighting panel, for use in a two or three lamp fluorescent fixture, which permits wider spacing of such fixtures in a room while producing greatly improved lighting throughout the room regardless of the fixture spacing.
Considerations of conservation have recently underscored the desirability of improving existing lighting systems without adding lamps, of reducing the number of lamps utilized in existing systems, and of designing new systems with a minimum of lamps. It is also increasingly recognized that the quality of light for many tasks may not be seriously impaired, and indeed may be enhanced, by a reduction in the quantity of artificial light provided for the task. Limiting lighting systems to moderate footcandle levels (say 50 footcandles for general office use) or low energy consumption (say 2 watts or less per square foot) is now being proposed.
Lighting systems are needed which reduce direct glare and veiling reflection, which are highly efficient, and which may be widely spaced and still provide uniform lighting throughout a work area. It is also desirable that such lighting systems be adapted to improving existing systems, and that in new or updated installations they may be economical to install and maintain.
The comfort of a lighting system is largely dependent on the amount of direct glare experienced by a viewer in a particular position relative to the elements of the lighting system. A well-designed lighting system should have low luminances at high viewing angles and should have low maximum-to-average luminance ratios.
Although two different types of lighting systems may produce equal footcandles, the visibility created by one may be greater than the other. Equivalent sphere illumination (ESI) is the Illuminating Engineering Society method of expressing the effectiveness of footcandles. ESI is related to the contrast produced on the visual task. When the contrast is reduced because of veiling reflections, the effectiveness of footcandles is reduced also. Loss of contrast is produced when an excessive amount of light arriving on the task at a low angle (near the vertical) reflects into the viewer's eyes. A lighting system should ideally produce high ESI values for all viewing directions relative to the light sources.
The efficiency of a lighting system is, of course, vital to its success in conserving energy. The distribution of light from individual luminaires, however, is also important. Not only should the light distribution pattern increase the comfort and effectiveness of the luminaire, but it should also permit the luminaires of the system to be arranged in the most convenient and economical manner.
In many installations, the most convenient and economical arrangement of luminaires is in continuous parallel rows, spaced as far apart as necessary to produce the desired lighting levels. This arrangement permits the luminaires to be wired through the fixtures. However, it makes designing for decreased illumination levels very difficult. For example, a typical lighting system utilizing two foot by four foot troffers, each containing two 40 watt lamps, may arrange the fixtures in continuous rows spaced on eight foot centers to produce an average maintained illumination of about 100 footcandles. To reduce the average illumination to 50 footcandles, using the same arrangement, requires that the rows be spaced apart 16 feet on center. If the fixtures are mounted 10 feet above floor level, their mounting height is taken as 7.5 feet, the nominal working surface being taken as 30 inches. Therefore, 16 foot spacing yields a typical spacing-to-mounting height ratio of 2.1. Presently known troffers cannot produce an acceptably uniform illumination level at such high spacing-to-mounting height ratios. Although there are no formal standards, it is generally recognized that a ratio of maximum illumination to minimum illumination over about 1.3 is noticeable and objectionable.
Other arrangements of luminaires, such as broken rows, checkerboard patterns, and modular spacings, are also frequently used. It is therefore desirable that a lighting system also be sufficiently adaptable to provide improved lighting at various illumination levels utilizing any of a number of fixture arrangements other than continuous rows.
The various aspects of good lighting touched upon here are well known to those in the art and are discussed, for example, in the current edition of the Illuminating Engineering Society handbood and in more recent IES publications.